PHOENIX — The Orioles’ night ended Monday with Arizona Diamondbacks left fielder Adam Eaton hitting a ball into the swimming pool in right-center at Chase Field then and running into a pile of teammates at home plate.

On a night in which two teams chasing playoff spots traded five ties or lead changes in the final 2 ½ innings, the Orioles fell just short in a 7-6, walk-off series-opening loss in the desert.

The Orioles, who made baseball history by going 29-9 in one-run games on their way to the playoffs last season, are 14-19 in contests decided by one-run this season, including 4-12 on the road.

Eaton’s game-winning homer came off Darren O’Day’s first delivery of the ninth inning – a 78-mph slider – quickly ending the right-hander reliever's third straight day of work. But this loss came down to much more.

The Orioles bullpen – which had allowed just one run over its previous 14 innings over the first five games of this eight-game interleague NL West road trip – yielded five runs in 2 2/3-plus innings of relief.

“There’s a lot more things,” Orioles manager Buck Showalter said. “It’s unfortunate. But we had our opportunities. Guys kept grinding. Not every game everybody is operating at full capacity, so that’s life of being on the road and doing things to win baseball games. You can go back to every one-run game and find some things that were offset.”

The Orioles held a 4-2 lead going into the seventh before opening the inning with a hit batsman and a walk. Both of those runners scored – as did the go-ahead run.

“They got me,” said right-hander Tommy Hunter, who allowed two runs in the inning. “Bottom line is that the seventh inning was big and definitely a lot of weight falls on my shoulders. It is what it is. Come back tomorrow. These guys battled back and with another couple big hits but at the end of the day it comes down to the seventh inning. Then people don’t get into situations they did. Tough game.”

Earlier in the game, the Orioles appeared to end the second inning when Gerardo Parra tried to score from second on a passed ball and was easily tagged out by three steps at the plate by Scott Feldman. But third-base umpire Alfonso Marquez awarded the Diamondbacks the run, ruling that Orioles third baseman Manny Machado obstructed the runner as he turned around third.

“Well, it’s the wrong call,” Showalter said. “It’s pretty obvious. It’s a lapse in judgment. He was out by three steps. Manny would have had to lay out in front of the bag and block his way. … It’s something that unfortunate that in a one-run game.”

The Orioles (65-53) wasted an opportunity to pick up a half game on the American League East-leading Boston Red Sox and the second-place Tampa Bay Rays, who were both idle. Instead, they are now five games out of first and trail the Rays by two games for the second AL wild-card spot.

They also wasted first baseman Chris Davis’ majors-leading 43rd homer of the season — a game-tying shot off left-hander Joe Thatcher in the top of the eighth — and a gritty rally in the ninth in which they willed the tying run across against Arizona closer Brad Ziegler (6-1).

Davis' homer on Monday tied him with Rafael Palmeiro (1998) for the fourth-highest single-season home run total in Orioles history.

O’Day (5-2), who threw scoreless innings each of the previous two nights, made just one pitch Monday — one that Eaton crushed.

“Darren’s been nails for us all year and he will be again,” Showalter said.

Eaton’s homer came just after the Orioles battled back to scrape a run across in the top of the ninth, helped by a possible inning-ending double-play ball that first baseman Paul Goldschmidt couldn’t get out of his glove as he first looked toward second and then hesitated when throwing to first.

As quickly as Davis tied the game with an opposite-field blast into the left-field stands in the top of the eighth, the Diamondbacks regained the lead on Wil Nieves’ leadoff homer off left-hander Troy Patton in the bottom of the inning.

Before Nieves’ homer, the Orioles bullpen blew a two-run seventh inning lead as Arizona rallied for three runs for a 5-4 advantage.

Lefty Brian Matusz opened the seventh inning by hitting Arizona shortstop Didi Gregorius, and Hunter followed by walking the first batter he faced, pinch hitter Matt Davidson. Both runs ended up scoring, as did the go-ahead run on Martin Prado’s two-out single that knocked in Eaton.

“It’s just one of those games where it see-sawed back and forth and every time you thought you might have a lead you could hold onto, they put some good at-bats together and then we put some good at bats together,” said Roberts, who was 2-for-3 with two RBIs.. “It’s one of those games you certainly want to win, but you can’t win them all.”

Feldman — starting for the first time in nine days — allowed two runs (one earned) on four hits over 5 1/3 innings. He left after throwing 109 pitches.

Matt Wieters hit his 16th homer of the season, a towering solo shot over the center field fence off Arizona starter Wade Miley that gave the Orioles a 4-2 lead in the sixth.

Despite falling short of a quality start, Feldman became the seventh straight Orioles starter to allow three or fewer runs. Orioles starters have a 2.91 ERA in that span. Leading 3-2 in the fifth, Feldman overcame a leadoff triple by Arizona leadoff hitter A.J. Pollock, retiring the next three batters to prevent a run.

“That’s always big,” Feldman said. “You don’t like to be in those situations, but its baseball. Thirty percent of the time someone is going to be on base, so I was really fortunate to make some big pitches in some of those situations to keep us with the lead. It’s really something I hadn’t done in my previous two [starts]. If get in those situations and give up a big hit. It’s good step in the right direction with that.”